Characterisation of infrared radiative flux and flux divergence effects using a two-channel radiometer-application to cooling on calm and clear nights

This study examines the characteristics of longwave radiation in the nocturnal lower boundary layer atmosphere and its impact on nocturnal cooling. The approach uses field measurements from a purpose-built pyroelectric radiometer that obtains radiance measurements in two spectral channels simultaneously. One of those channels corresponded to a region of strong atmospheric activity and the other was a 'window' channel where atmospheric activity was negligible. Data from the instrument, in combination with predictive equation derived from results from the MODTRAN radiative transfer model, yield the full broadband radiation fluxes that are needed for the nocturnal cooling calculations. Results of a field measurement program are presented, including comparison of predicted and measured cooling rates on calm clear nights in May, June and July of 2002. Field measurements were made at two sites: Hobart Airport (Lat:-42,8;Lon:147.5) and the University of Tasmania campus in Hobart (Lat:-42.9; Lon: 147.3). Comparison of expected and measured results from the radiometer's field trial showed a consistent underestimation by the MODTRAN model of the upwelling radiation flux in the active spectral channel over short path lengths. The measured values were found to consistently exceed the values predicted by the model. Causes for this underestimation were explored by modeling various atmospheric conditions, without a conclusive result. Comparison of predicted and measured cooling rates yielded a favorable comparison in their trends and a reasonable quantitative comparison. The cooling trend was highest in the early evening and decreased as the evening progressed. Beyond midnight there was a reversal in the cooling, characterised by a brief warming period. Reasons for this process are discussed.